1. Field of the Invention
The present invention relates to an imaging apparatus for taking pictures of an object using a solid-state imaging device such as a CCD, a drive unit for driving the same solid-state imaging device and a driving method therefor.
2. Description of the Related Art
In recent years, various kinds of digital cameras for taking pictures of an object using a solid-state imaging device such as a CCD have been developed. There are various types of CCDs. FIG. 1 shows a typical CCD configuration. This CCD comprises two-dimensionally arrayed photoelectric conversion devices (photodiode PDs), a read gate, a vertical charge transfer line (VCCD), a horizontal charge transfer line (HCCD) and an output amplifier.
When vertical transfer of signal charge is executed through the VCCD according to four-phase driving method, one of four vertical charge transfer electrodes (V1, V2, V3, V4) provided for each photoelectric conversion device PD, for example, V2 is connected to a corresponding photoelectric conversion device PD through the read gate. The vertical charge transfer electrode V2 acts as a gate electrode of the read gate at the same time. When a charge transfer pulse (TG), which is a higher voltage driving pulse than an ordinary driving pulse for vertical charge transfer is applied to the vertical charge transfer electrode V2, electric charge can be transferred from the photoelectric conversion device PD composing pixels to the vertical charge transfer line VCCD.
An electrode position indicated as shading in FIG. 1 indicates that electric charge is currently accumulated in a channel under that electrode. Ordinarily, the VCCD is stopped in a period of the horizontal charge transfer, which executes transferring electric charge through the HCCD. Thus, in that period, signal charges of each pixel are accumulated and held in two serial vertical charge electrodes V2, V3 separately. Electric charges accumulated in the two vertical charge electrodes V2, V3 are shifted by four each in the vertical direction each time when vertical charge transfer is executed by a cycle and transferred to next vertical charge electrodes V2, V3 of the same phase.
FIG. 2 shows a conventional typical driving waveform for vertical transfer and horizontal transfer and FIG. 3 shows changes in potential of each channel under the vertical charge transfer electrodes V1, V2, V3, and V4 of this case.
Three kinds of voltages, a negative low voltage L, a positive middle voltage M and a high voltage H higher than the middle voltage M, are applied to the vertical transfer voltage V2. The low voltage L and the middle voltage M are used for vertical transfer in the VCCD. The high voltage H is used for electric charge transfer from the PD to the VCCD.
At time t=t1, a low voltage L is applied to the V1, V4 while the middle voltage M is applied to the V2, V3. Consequently, no potential well is formed under the electrodes V1, V4 and a deep well is formed under the electrodes V2, V3. As a result, channels under the electrodes V1, V4 become potential barriers to block mixing of noise signals from the surrounding. Next, at time t=t2, the voltage of the electrode V4 is raised from L to M. Then, a signal charge is moved from channels under the electrodes V2, V3 to channels under the electrodes V2, V3, and V4 and at time t=3, the voltage of the electrode V2 turns to L while the signal charge is moved to channels under the electrodes V3, V4. By carrying out processing for moving a potential well forming position in the vertical direction in succession, electric charge is moved to channels under the electrodes V2, V3 corresponding to a next photoelectric conversion device PD at time t=9.
Of electric charge transferred vertically as described above, electric charge existing in a channel under an electrode at a bottom end portion of the vertical charge transfer line is moved to the horizontal charge transfer line HCCD. At time t=10, horizontal transfer of electric charge of one line moved to the horizontal charge transfer line is carried out and read out of the imaging device through an output amplifier. By reading out all electric charges of the PD connected to the VCCD in succession, one frame is read out.
Of the four electrodes V1, V2, V3, and V4, the electrode V2 is connected to a read gate for transferring electric charge accumulated in the photoelectric conversion device PD to the vertical charge transfer line VCCD. Then, if a higher voltage H than the middle voltage M at the vertical charge transfer is applied to the electrode V2, electric charge accumulated in the photoelectric conversion device PD is transferred to the VCCD. When the voltage of the electrode V2 is M or L, the channel under the read gate forms a potential barrier thereby blocking electric charge accumulated in the photoelectric charge device PD from flowing into the VCCD. Here, a period of time t1 to t10 is called the horizontal blanking period.
In a first horizontal blanking period of each frame, the high voltage H is applied to the electrode V2 as electric charge transfer pulse TG at time t=9, so that a deep potential well is formed under the electrode V2. Consequently, the read gate turns conductive, so that electric charge is transferred from the PD to the potential well under the electrode V2. The electric charge transferred from the PD to the electrode V2 is held by the V2, V3 in the horizontal transfer period at time t=10 and shifted only by one line in a next horizontal blanking period in the vertical direction. Only the vertical charge transfer is carried out since the second horizontal blanking period but no electric charge is transferred from the PD to the VCCD.
However, the conventional driving system for transferring the signal charge vertically while accumulating and holding the signal charge in the vertical charge transfer electrodes V2, V3 needs to hold electric charges in the electrodes V2, V3 in the horizontal blanking period and a positive middle voltage M continues to be applied to the electrodes V2, V3.
In this case, because the electrode V2 or the read gate electrode is held at plus potential, electron or dark charge induced under the read gate electrode is increased, so that this dark charge flows into the PD thereby worsening signal-to-noise ratio. FIGS. 4A to 4C show this state. The dark charge means the charge which behaves as noise against signal charge.
FIGS. 4A, 4B, and 4C show potentials of a channel under the electrode V2 when the voltages applied to the electrode V2 is M, L, and H, respectively. Conditions in which the voltage applied to the electrode V2 is M, L, and H are expressed in V2=M, V2=L, and V2=H, respectively. When V2=M, a potential well is formed under the electrode V2 as shown in FIG. 4A. In this case, because the silicon substrate surface under the electrode V2 which is the read gate turns to plus potential, the dark charge on the silicon substrate surface under the electrode V2 is increased. Because in the above-described driving system, V2=M is maintained in a period in which the driving of the VCCD is stopped, the dark charge induced by the silicon substrate surface under the electrode V2 is likely to flow into the PD.
On the other hand, when V2=L, as shown in FIG. 4B, no potential well is formed under the electrode V2 and positive holes gathered in a channel under the electrode V2 couple with the dark charges again so that the dark charges decrease, thereby providing no problem.